Industrial equipment often require high-speed, high-precision simulation front-end schemes, and signal levels in the control system are typically one of the following categories: single-ended current (4 mA to 20 mA), single-ended differential voltage (0 V-5V, 0 V to 10 V, ± 5 V, ± 10 V) or small signal input from sensors such as thermocouple or weighing sensor. The large-scale voltage swing is also very typical, especially small signal differential input; therefore, good common mode suppression performance is an important feature of analog signal processing system.
The figure below shows that the analog front-end circuit is optimized to provide high-precision and high common mode rejection (CMRR) when processing these types of industrial grade signals.
The circuit is level conversion and attenuation of the signal, so that the signal can be compatible with the input range of most modern single power SAR ADCs, such as high performance, 16-bit 250 KSPS PULSAR® ADC AD7685. For the input signal of 18 V P-P, the common mode suppression (CMR) performance of the circuit is approximately 105 dB (100 Hz) and 80 dB (5 kHz).
High precision, high input impedance and high CMR are provided by the meter amplifier AD8226. For high-precision applications, it is necessary to have high input impedance to minimize system gain errors and achieve excellent CMR. The AD8226 gain can be programmed in the range of 1 to 1000 by resistors.
If the resistive level converter / attenuator stage is directly connected directly, a mismatch is blocked between the resistors, resulting in a decrease in CMR performance. The AD8226 can provide excellent CMR performance required for small signals and large signal inputs. No external components are required, the level converter / attenuator / driver AD8275 can perform attenuation and level conversion functions in this circuit.
Since the signal bandwidth is relatively low, the sigma-delta type ADC is typically used for high resolution measuring systems, and the σ-delta architecture can provide excellent noise performance under low update rate conditions. However, in more and more designs, especially multi-channel systems, the update rate is constantly increasing to update each channel or increase channel density faster. In this case, high performance SAR ADC is a good alternative. The circuit shown above uses 250 KSPS 16-bit ADC AD7685, high-performance meter amplifier AD8226 and attenuator / level converter / amplifier AD8275 and configured as a complete system solution, no external components are required.
The AD8226 is based on the traditional three-way implantation structure. This topology consists of two levels: the first level provides a differential amplifier preamplifier, which will then be a differential amplifier that eliminates common mode voltage. The above figure shows the simplified schematic of the AD8226.
The first stage operates as follows: to maintain a constant voltage on the bias resistor RB, A1 must keep the node 3 to maintain a stable diode pressure drop than the positive input voltage. Similarly, A2 must maintain a constant diode pressure drop on the node 4 on the negative input voltage. Therefore, the differential input voltage is copied to the gain setting resistor RG. Currents flowing through this resistor must also flow through resistors R1 and R2, which generates a differential signal of gain adjustment between the A2 and A1 outputs. Note that as an additional product of the differential signal obtained by gain, the original common mode signal of the drowning diode pressure drop voltage still exists.
The second stage is a differential amplifier, consisting of A3 and 4 50 kΩ resistors. The role of this stage is to eliminate common mode signals on the zoomed differential signal.
This circuit is built into a rail-to-rail output meter amplifier AD8226 and is connected to the positive input of g = 0.2 differential amplifier AD8275, and the output of the differential amplifier is connected to 16 bits, 250 KSPS, PULSAR in MSOP / QFN package The input of the ADC AD7685. The gain of the AD8226 is set to 1 (high voltage / current input), and its output is referenced. You can use single-ended or differential inputs. The output of the AD8226 is a bipolar signal for driving the AD8275 input. The AD8275 is used to attenuate and level conversion of the bipolar input, thereby providing
Gain of 0.2. Therefore, when the differential signal of 20 V P-P is input thereof, the output will generate a single-ended signal of 4 V P-P. 4.5 V Precision Benx Voltage Source ADR439 is used to provide an internal common mode bias voltage (VREF / 2 = 2.25V) for the AD8275, and provide an external reference voltage for the AD7685 ADC. Under these conditions, the output swing of the AD8275 is +0.25 V to +4.25 V, located within 0 V to +4.5 V of the AD7685.
The ADP1720 is used to provide 5 V power supplies for AD8275 and AD7685. The reason for choosing ADP1720 is because it has a high input voltage range (up to 28 V). In this circuit, the ADP1720 only provides a current of approximately 4 mA of AD8275 and AD7685, so in worst case, the power consumption of the regulator is approximately 90 mW when the 28 V input is input, which makes the entire system can use external ± 15 V. Power supply.
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